Since plastic materials are light in weight, excellent in toughness, and can easily be dyed, they are recently frequently applied to various optical materials. Properties that are particularly required for many optical materials include a high refractive index. As optical materials having a high refractive index, a number of episulfide compounds that can give optical materials with a refractive index of 1.7 or higher have been found (see Patent Documents 1, 2 and 3). As other technique for increasing a refractive index of a plastic material, a method in which inorganic particles are dispersed into an organic resin is well known. According to this method, however, haze is caused since light scattering caused by the particles cannot be avoided completely. Thus, use of an episulfide compound is more preferable in terms of light transmittance.
Since the method for curing a composition using an episulfide compound is mostly heat curing, there is large restriction on the usage thereof. Furthermore, a photocurable composition is also strongly desired for improving productivity. For example, in order to impart an optical function to a transparent substrate having a refractive index exceeding 1.73 such as lanthanum glass or sapphire glass, a photocurable composition that has a refractive index exceeding 1.73 and that can be provided with a fine structure is desired.
Patent Document 4 reports a cured resin product with a high refractive index that is obtained by photocuring an episulfide composition containing a photobase generator.
As a method for obtaining a cured resin product with a higher refractive index, a technique of copolymerizing a cyclic sulfur compound and an episulfide compound is known. Patent Document 5 reports that a sulfur-containing episulfide compound can be cured with heat and Patent Document 6 reports that an episulfide compound containing a cyclic skeleton structure compound having sulfur can be cured, to obtain a transparent resin having a refractive index exceeding 1.73, respectively. It is, however, generally difficult to use a cyclic sulfur compound for a photocurable composition with a high refractive index. Specifically, a typical cyclic sulfur compound S8 quenches optically active species due to the strong radical-inhibiting activity attributed to its polysulfide structure, while a cyclic sulfide compound that contains more carbon atoms than sulfur atoms is poorly effective in enhancing the refractive index.
Moreover, Patent Document 6 shows only an example of thermal polymerization and not photocuring, and thus it has no intension of using a photopolymerization initiator. Therefore, the property of inhibiting photoactive species is not considered at all.